1. Field of the Invention
The present invention relates to an apparatus and method used for OFDM (Orthogonal Frequency Division Multiplexing) based mobile communications.
2. Description of the Related Art
A conventional communication apparatus used for OFDM-based mobile communications (hereinafter referred to as xe2x80x9cOFDM communication apparatusxe2x80x9d) is explained using FIG. 1 and FIG. 2. FIG. 1 is a block diagram showing a configuration of a conventional OFDM communication apparatus and FIG. 2 is a schematic diagram showing a frame format in an OFDM-based radio communication.
First, the configuration of the conventional OFDM communication apparatus is explained using FIG. 1. A baseband signal is input to quasi-coherent detector 101. This baseband signal is a signal received from an antenna which is not shown in the diagram and then subjected to normal radio reception processing by a radio reception section which is not shown in the diagram. Quasi-coherent detector 101 is controlled by a local signal output from oscillator 114 which will be described later and performs quasi-coherent detection on the input baseband signal. LPF (analog low-pass filters) 102 and 103 eliminate an unnecessary frequency component of the signal subjected to quasi-coherent processing. A/D converters 104 and 105 convert the analog signal with the unnecessary frequency component eliminated to a digital signal.
FFT (Fast Fourier Transform; hereinafter referred to as xe2x80x9cFFTxe2x80x9d) circuit 106 performs FFT processing on the A/D-converted signal using the output signal of timing generator 116 which will be described later as a trigger signal. Demodulation section 107 demodulates the FFT-processed signal. Determination section 108 determines the demodulated signal.
Delay circuits 109 and 110 delay the A/D-converted signal. Complex multiplier 111 performs complex multiplications using the A/D-converted signals and delayed signals. Accumulator 112 accumulates the complex multiplication result of complex multiplier 111 and outputs the accumulation result to maximum value detector 115 and frequency offset detector 113.
Maximum value detector 115 detects a maximum value of the accumulation result of accumulator 112. When maximum value detector 115 detects the maximum value, timing generator 116 outputs a signal to start FFT processing to FFT circuit 106.
Frequency offset detector 113 calculates a frequency offset necessary for frequency offset compensation using the accumulation result of accumulator 112 and outputs the calculation result to oscillator 114. Oscillator 114 outputs a local signal with frequency offset compensation to quasi-coherent detector 101.
Then, the operation off the conventional OFDM communication apparatus is explained. A signal input via an antenna which is not shown in the diagram is subjected to normal radio reception processing by a radio reception section which is not shown in the diagram and converted to a baseband signal. This baseband signal is subjected to quasi-coherent detection processing by quasi-coherent detector 101. The baseband signal subjected to quasi-coherent detection processing by quasi-coherent detector 101 is stripped of an unnecessary frequency component by LPF 102 and 103, converted to a digital signal by A/D converters 104 and 105, and becomes a digital baseband signal.
The digital baseband signal is subjected to FFT processing by FFT circuit 106 where a signal assigned to each sub-carrier is obtained. The signal processed by FFT circuit 106 is demodulated by demodulator 107, determined by determination section 108 and becomes a demodulated signal.
On the other hand, the communication apparatus in an OFDM-based mobile communication needs to provide timing so that FFT is started with symbol synchronization established with a base station, the transmitting side.
The following is an explanation of how symbol synchronization is established.
In an OFDM-based mobile communication, symbol synchronization is generally established using a synchronization symbol inserted after an AGC (gain control) symbol of each symbol and a phase reference symbol which is identical to the synchronization symbol as shown in FIG. 2. The phase reference symbol is followed by a guard segment and valid symbol.
First, complex multiplier 111 performs complex multiplications on signals before FFT processing and other signals before FFT processing which have been delayed by one symbol (unit symbol) by delay circuits 109 and 110.
Then, accumulator 112 accumulates the output of complex multiplier 111. Since the synchronization symbol and the phase reference symbol have the identical waveform as described above, the accumulation result shows a peak at the start of each guard segment. Maximum value detector 115 detects the accumulation result at this peak. Then, a signal indicating that maximum value detector 115 has detected a maximum value is sent to timing generator 116. A signal to start FFT processing is sent to FFT circuit 106 by timing generator 116 that has received this signal. FFT circuit 106 receives the signal from timing generator 116 and starts FFT processing.
Through the operation described above, the communication apparatus in the OFDM-based mobile communication can establish symbol synchronization and provide FFT start timing.
Furthermore, since an OFDM-based mobile communication is greatly affected by deterioration of the reception characteristic due to a frequency offset, frequency offset compensation is carried out. The operation of frequency offset compensation is explained below.
In an OFDM-based mobile communication, frequency offset compensation is generally carried out using the synchronization symbol and phase reference symbol shown in FIG. 2.
First, as described above, complex multiplier 111 performs complex multiplications on signals before FFT processing and other signals before FFT processing which have been delayed by one symbol (unit symbol) by delay circuits 109 and 110. Accumulator 112 accumulates the complex multiplication results and sends the result to frequency offset detector 113.
Frequency offset detector 113 calculates the amount of phase rotation using the accumulation result of accumulator 112 and calculates a frequency offset from this amount of phase rotation. This frequency offset is sent to oscillator 114.
Using the frequency offset sent from frequency offset detector 113, oscillator 114 generates a local signal with frequency offset compensation and sends it to quasi-coherent detector 101. Quasi-coherent detector 101 performs quasi-coherent detection under the control of the local signal sent from oscillator 114.
Through the operation described above, the communication apparatus in the OFDM-based mobile communication prevents deterioration of the reception characteristic due to a frequency offset.
However, the conventional apparatus has the following problems. That is, under a multi-path environment, as shown in FIG. 3, the OFDM communication apparatus receives n delay waves, delay wave 1 to delay wave n, in addition to a dominant wave. Thus, the synchronization symbol in the dominant wave receives interference by n AGC symbols of respective delay waves. That is, the synchronization symbol in the dominant wave receives interference because the synchronization symbol in the dominant wave has a time area overlapping with the AGC symbols of delay wave 1 to delay wave n.
Especially, if the delay time of each delay wave is short, the level of the delay wave is high, and therefore the synchronization symbol in the dominant wave, or more specifically, the first half of this synchronization symbol receives greater interference between symbols.
Thus, if the entire synchronization symbol is used for accumulation processing at the time of the aforementioned frequency offset compensation, the conventional apparatus has the problem of the accuracy of frequency offset detection deteriorating.
It is an objective of the present invention to provide an OFDM communication apparatus that improves the detection accuracy of a frequency offset in a multi-path environment.
This objective is achieved by using a part of a synchronization symbol that is less affected by interference between symbols.